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P R E S E N T E D BY :

K A R A N GO Y A L

P R E S E N T E D TO :

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H AE L SC H A W E N G E

Electrostatic precipitators: This electrical equipment was first introduced by Dr.F.G. Cottel in 1906 and was first economically used in 1937 for removal of dust and ash particles with the exhaust gases of thermal power plants

PRINCIPLES OF ELECTROSTATIC PRECIPITATOR

Electrostatic precipitation is a method of dust collection that uses electrostatic forces, and consists of discharge wires and collecting plates.

A high voltage is applied to the discharge wires to form an electrical field between the wires and the collecting plates, and also ionizes the gas around the discharge wires to supply ions.

When gas that contains an aerosol (dust, mist) flows between the collecting plates and the discharge wires, the aerosol particles in the gas are charged by the ions. The Coulomb force caused by the electric field causes the charged particles to be collected on the collecting plates, and the gas is purified.

This is the principle of electrostatic precipitation, and Electrostatic precipitator apply this principle on an industrial scale.

ESP OPERATIONElectrostatic precipitators use electrostatic charges to

separate particles from a dirty gas stream. High voltage, direct current electrodes are used to

establish a strong electric field. This field (known as a corona) delivers a (usually)

negative charge to particles as they pass through the device.

This charge forces the particles onto the walls of collection plates or tubes.

These collection surfaces (or collection electrodes) are then rapped, vibrated, or washed with water to dislodge the particles, which fall into a hopper.

IN SUMMARY, THE MAJOR COMPONENTS OF A PRECIPITATOR INCLUDE:

Collection electrodesDischarge electrodesHigh voltage power supplyPrecipitator controlsRapping or spray washing systemsPurge air systemsEfficiency losses occur in ESPs due to reentrainment, sneakage, and

back corona. Reentrainment occurs due to rapping, causing a small percentage (10-15 percent) to be projected back into the gas stream. Sneakage is the small part of gas flow that moves around the charging zones untreated due to practical design constraints. Back corona occurs when an electric field becomes large enough to cause an electrical breakdown which reduces the charge on particles.

ESP DESIGN

 To begin, a manufacturer will ask for a number of process variables which describe the conditions and requirements of the system. These variables include:

Gas flow rate - how fast the gas is moving through the system. At higher flow rates, particle reentrainment increases rapidly, but insufficient flow will result in poor gas distribution or particle dropout.

Particle size and size distribution - the average size and size distribution of PM in the gas flow. Larger particles pickup charge more easily, while an abundance of small particles may suppress the generation of the corona.

A typical ESP has thin wires called discharge electrodes (DE), which are evenly spaced between large plates called collection electrodes (CE).

Design factors that determine an ESP's performance include :

Particle resistivity - measure of the particles' resistance to electrical conductance. Particles with high resistivity have difficulty acquiring charge, while particles with low resistivity may lose their charge too easily and not stick to the collection plate. Resistivity is influenced by the particulate's chemistry and the gas temperature.

Gas temperature - the temperature of the gas flow in the system. Particle chemistry - the chemical makeup of the particulate

matter in the gas flow.Precipitator size - the size of the precipitator affects its

collection efficiency, footprint, and gas flow capacity. Power input - the power supplied to the system to induce the

electric field. Increasing power input improves collection efficiency under normal conditions.

Types of ESPs : ESPs are classified based on a number of different

factors, including the collector design, the number of stages, and whether the process is dry or wet.

 Plate or TubularThe functional design of an ESP incorporates either

plate or tubular collection surf Plate PrecipitatorsPlate ESPs primarily collect dry particles and are used

more often than tubular precipitators. They can have wire-plate or flat-plate electrodes.

Plate-Wire PrecipitatorsIn a plate-wire ESP, gas flows between parallel plates of

sheet metal and high-voltage long metal wires. It allows many flow lanes to operate in parallel, making it suitable for handling large volumes of gas. 

Plate-wire precipitators are among the most common types of ESPs. In industry, they are used in cement kilns, incinerators, boilers, cracking units, sinter plants, furnaces, coke oven batteries, and a variety of other applications.

Flat Plate Precipitators: Smaller precipitators use flat plates instead of wires for high-voltage electrodes. The flat plates increase the average electric field used to collect particles and provide additional surface area for particle collection. They are less susceptible to back corona than conventional plate-wire precipitators but also have higher rapping losses.

Flat plate ESPs can be used in applications with high-resistivity particles with small (1 to 2 µm) diameters. Fly ash can be captured using flat plate ESPs, but typically requires low velocities to prevent significant rapping losses.

FLAT ESP

Tubular PrecipitatorsTubular ESPs consist of parallel arrangements

of tubes with high-voltage electrodes running on their axis. The tubes may be arranged as a circular, square, or hexagonal honeycomb with gas flowing upwards or downwards. They are designed as one-stage units in which all the gas passes through the tube, eliminating sneakage. They are still susceptible to inefficiencies from corona non-uniformities.

Tubular precipitators are less common than plate types. They are used in applications involving wet or sticky particulate, and are typically cleaned with water for lower reentrainment losses than typical ESPs. They also can be tightly sealed to prevent leakage of material, an important consideration for valuable or hazardous substances.

TUBULAR ESP

Single or Two StageESPs can be designed as either single or

two stage configurations. Single-Stage PrecipitatorsMost industrial scale ESPs are single

stage. They use very high voltages to charge particles and incorporate charging and collection together in the same stage. Sets of electrodes and collector surfaces (plates or tubes) operate in parallel to each other.

Two -Stage PrecipitatorsTwo-stage ESPs operate in series rather than parallel

configuration. Instead of using a side by side design, they incorporate separate particle charging and collection stages. This allows more time for particle charging, less susceptibility to back corona, and economical construction for smaller sizes.

Two-stage precipitators are separate and distinct from other ESPs, originally designed for air purification in conjunction with air conditioning systems. They are typically used for smaller, lower-volume applications. They are usually applied to submicron sources emitting oil mists, smokes, fumes, or other liquid aerosols. Many are sold as pre-engineered, package systems.

TWO STAGE ESP

DRY OR WET ESPs can also be classified based on whether they operate using

a dry or wet process.Dry ESPsDry electrostatic precipitators are used to capture particles in dry

product streams. They use periodic rapping to separate the accumulated dust from the collector plates and discharge electrodes. The dust layer (released by rapping) is collected in a hopper and then removed by an ash handling system. Typically, rapping will also project some of these particles (around 10-15 percent) back into the gas stream (known as reentrainment).

Dry electrostatic precipitators are often not suitable for submicron particulate applications because of particle size, resistivity, and other issues

WET ESPS (WESPS)Wet electrostatic precipitators are used to strip wet (saturated) gas

streams of particles. They use water sprays to condition/trap particles for collection and also to clean the particles off collection surfaces. WESPs collect particulate matter not suitable for dry ESPs, including sticky, moist, flammable, explosive, or high resistivity solids. WESPs can also remove very fine (submicron) particulate that dry ESPs cannot capture effectively. The use of water also gives these devices gas scrubbing capabilities. Most wet precipitators are tubular designs. 

However, WESPs are more costly than dry ESPs. Because they incorporate water and corrosive gases, they must be designed from more expensive corrosion-resistant materials. Another disadvantage of WESPs is that the PM is collected as a slurry instead of a dry solid. This form is unsuitable for high value or recyclable materials and is more expensive to handle and dispose. If the water is being recycled and reused, the system also must incorporate a water purification step.

APPLICATIONSESPs may be specifically designed to meet the needs of certain industries or

applications. Some applications and media types include:Abrasives - baghouse fabrics are designed to withstand and capture abrasive

particles.Coolant and oil mists - unit is capable of filtering coolant smoke and mist from

metal finishing and forming processes, and machining oil mists.Explosive media - unit is capable of filtering explosive dusts, mists, and/or fumes.Fine powders - unit is capable of filtering fine powders such as carbon black, talc,

pigments, oxides, and plastic compounding dusts.Metalworking chips & fluids - unit can capture aerosols and fumes emitted

by metalworking fluids, including oils, lubricants, and coolants.Toxic media - unit is capable of filtering toxic materials such as dust, mist, fume,

or smoke from the air.Welding fumes - unit is designed specifically for the collection of welding fumes or

dust; these may include flux recovery systems. used in Power/Electric Cement Chemicals Metals Paper.

Depending on the design, ESPs are capable of handling large gas volumes across a wide range of temperatures, pressures, dust volumes, and acid gas conditions.

In the case of an ESP, a negative, high-voltage, pulsating, direct current is applied to the DEs in order to create a negative electric field and induce ionization of the passing PM.

. For the sake of understanding the charging process, the negative electric field can be mentally divided into three regions. The first region is right next to the discharge electrode, where the field is the strongest. The second region includes space between the DE and CE called the inter-electrode region, and is weaker than the first. The third region is located near the collection electrode and has the weakest field strength of all.

RESISTIVITY OF THE PARTICLES

•Particulate resistivity is probably the most important basic variable influencing the precipitator and therefore is an important design consideration.A too high level of electrical resistivity or too low level causes collection difficulty. A high resistivity dust, such as sulphur, does not readily give up its negative charge and assumes a positive charge. This causes the particulate to be repelled back into the gas stream of negatively charged particles. A low resistivity dust can be collected and repelled in this manner many times before finally being emitted to the atmosphere. Therefore, the presence of large quantities of carbon in the ash can adversely affect the collection efficiency of a precipitator. One thumb rule followed by designer is to downgrade the efficiency of the unit by 1% for every 1% of carbon in the gas over 15%. Therefore, one always wishes a medium resistivity for good collection efficiency. In coal fired boilers, sulphur in the form of SO2 affects resistivity. Resistivity has two components, one related to the bulk of the material and another is related to the surface of the particle, absorbed layer of gas. As the temperature increases, the absorbed surface contaminants evaporate and surface resistivity increases. And with all insulating materials, the volume resistivity increases with decreasing temperature.

Six steps typically take place:Ionization – Charging of particlesMigration – Transporting the charged particles to the collecting

surfacesCollection – Precipitation of the charged particles onto the

collecting surfacesCharge Dissipation – Neutralizing the charged particles on the

collecting surfacesParticle Dislodging – Removing the particles from the collecting

surface to the hopperParticle Removal – Conveying the particles from the hopper to a

disposal point

The major precipitator components that accomplish these activities are as follows:

Discharge ElectrodesPower ComponentsPrecipitator ControlsRapping SystemsPurge Air SystemsFlue Gas Conditioning / Sorbent Injection Systems

CORONA DISCHARGE:FREE ELECTRON GENERATION

The first region is where the particle charging process begins, and in this small area immediately surrounding the discharge electrode several things happen very rapidly (in a matter of a millisecond). As voltage applied to the DE is increased, it eventually reaches a point when the electric field around the conductor is high enough to form a conductive region, but not high enough to cause electrical breakdown or arcing to nearby objects. This phenomenon is commonly referred to as corona discharge and can be seen by the human eye as a luminous blue glow surrounding the DE. As free electrons created by the corona discharge are repulsed by the negative electric field, they move faster and faster away from the DE. This acceleration causes the electrons to literally crash into passing gas molecules and occasionally knock off some of their electrons. As these gas molecules lose electrons that are negatively charged, they become positively charged ions. So, this is the first thing that happens – gas molecules are ionized, and electrons are liberated. All this activity occurs very close to the discharge electrode, and as the process continues, it creates more and more free electrons and positive ions. The name for all of this electron generation is avalanche multiplication.

IONIZATION OF GAS MOLECULES

As electrons leave the strong electrical field region surrounding the discharge electrode, they enter the inter-electrode region where they begin to lose energy and slow down. Though there are still gas molecules in the inter-electrode region, the electrons kind of bump up to them and get captured instead of violently colliding with them, creating negative gas ions. Now we have ionization of gas molecules happening near the discharge electrode as well as in the inter-electrode area, but with a big difference. The ions created near the discharge electrode are positive and remain in that area. However, because the ions created in the inter-electrode area are negative, they want to move with the electrons in the direction opposite the strong negative field.

CHARGING & MIGRATION

Before PM can be captured, it must first acquire a negative charge and the negative gas ions created in the inter-electrode region play a crucial role in this process. When PM and negative gas ions cross paths, the gas ions stick to the particles and impart a negative charge to them. At first the charge is fairly insignificant, as most particles are huge compared to a gas molecule, but many gas ions can fit on a single particle, and they do. Small particles (less than 1 μm diameter) can absorb “tens” of ions, while large particles (greater than 10 μm) can absorb “tens of thousands” of ions. Eventually, there are so many ions sticking to the particles, that the particles begin to emit their own negative electric field. When this happens, the negative fields surrounding the saturated particles start to repulse negative gas ions and no additional ions are acquired. This is called the saturation charge and is responsible for inducing the PMs inescapable pull of electrostatic attraction, or migration. Bigger particles have a higher saturation charge and are consequently pulled more strongly to the collection plate than smaller particles that have a smaller saturation charge. Regardless of size, the particles eventually encounter the CE and stick due to adhesive and cohesive forces.

THEORY OF OPERATIONS: REMOVAL

Particulate matter that has accumulated to a certain thickness on the CE is removed by one of two processes, depending on the type of CE used. Collection electrodes in ESPs can be either plates or tubes, with plates being the more common of the two. Tubes-type ESPs are usually cleaned by water sprays, while plates can be cleaned by either water sprays or a process called rapping. We will focus on the latter.

RAPPING: DISLODGING PARTICULATE MATTERRapping is a process whereby deposited, dry particles are

dislodged from the CE by sending mechanical impulses, or vibrations, to the plates. Precipitator plates are rapped periodically while maintaining the continuous flue-gas cleaning process. In other words, the plates are rapped while the ESP is on-line; the gas flow continues through the ESP and the applied voltage remains constant. Plates are rapped when the accumulated dust layer is relatively thick (0.08 to 1.27 cm) so that the dust layer is coaxed to fall off the plates as large aggregate sheets instead of small sections, helping to minimize re-entrainment. Most precipitators have adjustable rappers that allow rapper intensity and frequency to be changed according to the dust concentration in the flue gas. Precipitator fields with heavy dust concentrations require more frequent rapping than fields with light dust concentrations.

RECYCLE OR DISPOSAL

As PM is dislodged from CEs, is falls into the hopper. A hopper is a dedicated collection bin with sides sloping approximately 50 to 70o so that PM is allowed to flow freely from the top of the hopper to the discharge opening in the bottom. Particulate matter collected in hoppers should be removed as soon as possible in order to avoid packing that is very difficult to remove. Most hoppers are emptied by some type of discharge device and then a conveyor transports the collected PM to its final destination for recycling or disposal.

In an ESP using liquid sprays to remove accumulated PM, the sludge collects in a holding basin at the bottom of the vessel. The sludge is then sent to settling ponds of lined landfills for proper ultimate disposal. Spraying can occur while the ESP is on-line and is typically intermittent. While water is generally used as the spraying liquid in wet ESPS, other liquids could be used if absorption of gaseous pollutants is also a goal.

Advantages of electrostatic precipitator•

This is more effective to remove very small particles like smoke, mist and fly ash. Its range of dust removal is sufficiently large (0.01 micron to 1.00 micron). The small dust particles below 10 microns cannot be removed with the help of mechanical separators and wet scrubbers cannot be used if sufficient water is now available. Under these circumstances, this type is very effective.

• This is also most effective for high dust loaded gas (as high as 100 grams per cu. meter)

• The draught loss of this system is the least of all forms(1 cm of water)

• It provides ease of operation.

• The dust is collected in dry form and can be removed either dry or wet.

COLLECTION EFFICIENCY: The weight of dust collected per unit time divided by the weight of dust entering the precipitator during the same unit time expressed in percentage. The computation is as follows:

(Dust in) – (Dust out) Efficiency =             (Dust in)         X 100

DISADVANTAGES OF A ELECTROSTATIC PRECIPITATOR

•The direct current is not available with the modern plants, therefore considerable electrical equipment is necessary to convert low voltage (400 V) A.C to high voltage (60000 V) D.C. This increases the capital cost of the equipment as high as 40 to 60 cents per 1000 kg of rated installed steam generating capacity.

• The running charges are also considerably high as the amount of power required for charging is considerably large.

• The space required is larger than the wet system.

• The efficiency of the collector is not maintained if the gas velocity exceeds that for which the plant is designed. The dust carried with the gases increases with an increase of gas velocity.

• Because of closeness of the charged plates and high potential used, it is necessary to protect the entire collector from sparking by providing a fine mesh before the ionizing chamber. This is necessary because even a smallest piece of paper might cause sparking when it would be carried across adjacent plates or wires

Factors affecting the performance of E.S.P.:

The present trend in adopting the gas cleaning device is to discharge the clean gas without containing SO2 to the atmosphere. One solution to this problem is to burn fuels containing less sulphur, but unfortunately low sulphur fuels are costly to use. However, in most cases burning low sulphur fuel increases the electrical resistivity of fly ash, particularly at low temperatures. This higher and unpredictable resistivity at low temperatures coupled with high collection efficiencies demand can spell trouble for low temperature precipitators. That's why pollution engineers are leaning towards precipitators operating at about 345 degrees where resistivity is not dependent on sulphur level in the flue gases.The principle of electrostatic precipitator is described in 3 stages as charging of the suspended particles, collecting of particulates under the influence of electrostatic field and removal of the precipitate from the collector plate. 

Many factors influence these three fundamental steps but they are critical to the reliability and performance of high temperature precipitators which are listed below:

CORONA CHARACTERISTICS: • Initiation of corona depends upon free electrons by random sources such as

natural radioactivity. Under the influence of an electrical field, these electrons are accelerated to a terminal velocity. The rapidly moving electrons produce additional free electrons y colliding with the orbital electrons of gas molecules and by ionization. At higher temperatures, flue-gas density is reduced, resulting in a reduced starting potential. Thus, at higher temperatures, lower voltages initiate the corona to start the precipitation process, resulting in more collection for a given voltage than at lower temperatures.

Electrostatic precipitators operated at maximum power input have steep corona characteristics; that is, the rate of change of corona current is much greater than the concurrent charge in precipitator-circuit voltage. The steeply rising corona current is further enhanced by increasing temperature of the stack gases. The net effect is to maximize power levels to achieve high efficiency.

RAPPING BEHAVIOURThis is perhaps the most complex among the three performance steps.

Non electrical adhesive forces which play a significant role in plate rapping, vary inversely with particle diameter, but depend generally on the chemical and physical nature of the particle. Moisture can increase adhesion at lower temperatures. Particle resistivity has a critical effect on the electrical force causing particles to slick to the collection plates: the more resistive the particle, the greater the force. Operation at low temperatures and high resistivity requires considerably more rapping acceleration on the collection plates than it does under normal resistivity, and higher temperatures. 

Conventional practice limits maximum average gas velocity in high resistivity and low temperature operation to approximately 1.2 m/s. this limit avoids losses due to re-entrainment of particles which can occur when the dust layer is dislodged violently. In contrast, precipitators run at 1.7 m/s gas velocity at higher temperature.

GAS VELOCITY• There are two forces acting on a particle having direct right

angles to each other. First is due to the flow of gas and second is produced by the electric force on the ionized particle perpendicular to the motion of the gas. The path followed by the particle will take direction which is resultant of the two forces mentioned above. Therefore the efficiency of the collector decreases with an increase in velocity which can be compensated by increasing the voltage supplied to the plates.

 BPA Quality Air Solutions LLC - Electrostatic Precipitation for Dust Collection EPA - Electrostatic Precipitator Operation (pdf) Infohouse - Electrostatic Precipitators (pdf) Neundorfer - Electrostatic Precipitator

KnowledgeBase

http://www.neundorfer.com/knowledgebase-posts/introduction-to-esp/